Removal of copper containing incrustations from ferrous surfaces



United States Patent "ice 3,438,811 REMOVAL OF COPPER CONTAININGINCRUSTA- TIONS FROM FERROUS SURFACES Lester W. Harriman, Angleton, andPaul E. Muehlberg, Jackson, Tex., and Fred N. Teumac, Charlotte, S.C.,assignors to The Dow Chemical Company, Midland, Mich., a corporation ofDelaware No Drawing. Continuation-impart of application Ser. No.387,481, Aug. 4, 1964. This application Oct. 18, 1965, Ser. No. 497,530

Int. Cl. B08b 3/08 US. Cl. 1342 9 Claims This application is acontinuation-in-part of US. patent application Ser. No. 387,481, nowabandoned.

This invention concerns the removal of plated copper from a ferroussurface. 7

It has long been desired to be able to remove plated copper from aferrous suf-ace, e.g., steel, without also oxidizing excessively theiron thereof. This problem has been especially difficult andeconomically costly in the removal of plated copper from the internalmetal surfaces of steam generating equipment, particularly high pressuresteam generating equipment, which is operated in connection with acondenser, the condensing surfaces of which are of copper alloy.

In the operation of high pressure steam generating equipment (over 600pounds per square inch steam pressure) in which the feed water islargely returned condensate from a copper alloy condenser, incrustationsare usually produced upon the steam generating surfaces of the steamgenerator despite the fact that the feed water is substantially pure.These incrustations oftentimes contain copper, both in metallic form andcombined with oxygen, corroded from the copper alloy condenser by theaction of the condensed steam which carries the copper to the steamgenerator.

Attempts to remove such incrustations, as by the use of conventionalacidizing procedures, are not wholly suc cessful. Tests have shown thatby acidizing incrusted steam generating surfaces of the usual highpressure steam generator, having copper in the incnustations, some ofthe copper is removed from the incrustations and some of the copperso-removed is redeposited elsewhere on the surfaces of the steamgenerating equipment during the acidization so that only a partial netremoval of copper from the incrusted surfaces results. Insofar as isknown, there is no completely satisfactory method commercially availablefor treating the internal ferrous metal surfaces of high pressure steamgenerating equipment subject to deposition of copper-containingincrustations so as to free the surfaces of the incrustations and thecopper. Accordingly, it is an object of the invention to provide amethod fulfilling this need. Other objects and advantages will becomeapparent as the description of the invention proceeds.

It has now been discovered that an aqueous solution of a ferric chelateof a polycarboxylic acid chelating agent alone or together with somefree polycarboxylic acid chelating agent when adjusted to an alkalinepH, i.e., in excess of 7 and up to about 10, by combination withammonia, an amine or a hydroxyalkylamine or one or more of the precedingin amount of at least 50 mole percent in combination with up to 50 molepercent of an alkali metal hydroxide, is quite effective in dissolvingplated copper from ferrous surfaces, e.g., of high pressure steam boilersurfaces.

3,438,811 Patented Apr. 15, 1969 The chelating agents used in thepractice of this in vention as their ferric chelates and, if desired, incombination with ammonium and amine salts of the aforementionedpolycarboxylic acid chelating agents are those of alkylene polyaminepolyacetic acid (APAPAA), e.g., ethylenediaminetetraacetic acid -(EDTA),N hydroxyethylethylenediaminetriacetic acid (NHEDTA); nitrilotriaceticacid (NTA) and N-2hydroxyethyliminodiacetic acid (OHEtIDA);diethylenetriaminepentaacetic acid (DTPA); and mixtures thereof,hereinafter referred to broadly as polycarboxylic acid chelating agents.

In practice, a ferrous metal surface on which copper has plated out,e.g., that of a high pressure boiler, is heated at a temperature aboveroom temperature and up to about 300 F. in the presence of an aqueoussolution of a ferric chelate of a polycarboxylic acid chelating agent,if desired also containing free chelating agent, which solution isadjusted to an alkaline pH up to about 10 with ammonia and/or an amineor alkanolamine and with up to a 50 mole percent proportion of an alkalimetal hydroxide, if desired. The resulting salts will here inafter bereferred to as ammonium and amine salts of said chelating agents. Asolution containing a total of about 0.5 weight percent polycarboxylicacid as iron chelate and, if desired, as ammonium and/or amine salt ofsaid polycarboxylic acid up to a saturated solution thereof can be used.The weight proportion of iron chelate of the total of free and chelatedchelating agent, i.e., degree or percent spentness, can be varied frombetween about 60 to about percent.

It appears that the solution containing ferric chelate oxidizes thecopper metal to copper ions (Cu++ and Cu which react with the resultingferrous chelate or with the free, i.e., uncomplexed or salt-form,chelating agent therein to form a copper chelate, and after a sufficientreaction time, as determined by analysis of the treating solution, theplated copper is dissolved. However, we

do not wish to be bound by this theory. As reaction proceeds, the ferricchelate isreduced to a ferrous chelate. This reduction wouldprogressively slow the copper-dissolving reaction. In order to maintaina useful level of ferric chelate, i.e., some of the iron chelate must bein the ferric form, it has been found necessary to add an oxidizingagent to the copper-dissolving ferric chelate-containing solution sothat ferrous chelate formed when the plated copper is oxidized to copperions is reoxidized to ferric chelate for continued oxidation andsubsequent dissolution of copper. This may be done by continuously orperiodically monitoring or analyzing the copper-dissolving solution andadding an oxidant such as hydrogen peroxide, water-soluble salts such asalkali metal or ammonium nitrites, permanganates, persulfates, orperchlorates; or such gaseous oxidants as nitrogen tetraoxide, oxygen orair, advantageously by a sparger, in amount suflicient to maintain someof the iron chelate in the ferric chelate form. Of these oxidants, airis preferred, since it does not substantially affect pH and itintroduces no extraneous matter.

The ferric chelates of polycarboxylic acid chelating agents useful inthe practice of the present method are advantageously made by reactingiron, iron oxide or hydroxide or magnetite with a polycarboxylic acidchelating agent which has been adjusted to an alkaline pH up to about 10with ammonia and/or an amine as stated above or with a mixture ofammonia and/or an amine and an alkali metal hydroxide, in proportions asspecified, so that an average of not more than one free carboxylic acidgroup remains per mole of chelating agent, at least about half of thecarboxylic acid groups of the chelating agent are in the ammonium oramine salt form and provided that an average of not more than half ofthe carboxylic groups are in the alkali metal salt form. Alternatively,the corresponding ferrous chelates are made and oxidized, at leastpartially to the ferric chelate form in the manner previously described,advantageously in use. It is not required that pure ferric and/orferrous chelates be used. On the contrary, a commercially attractiveiron chelate-chelating agent solution can be prepared by dissolvingiron-containing scale from ferrous surfaces, e.g., those of oxide scaledferrous boiler tubes, by the reaction with an aqueous solution of apolycarboxylic acid chelating agent adjusted to an alkaline pH withammonia and/or an amine or mixture thereof or with ammonia and/or anamine and with an alkali metal hydroxide, thereby forming iron chelatecontaining both ferrous and ferric chelate. Such a method is describedin copending US. patent application Ser. No. 296,464, filed July 22,1963 now United States Patent 3,308,065.

The more preferred ammonium and/or amine salts whose ferric chelates areused in the process of this invention are those of the APAPAAs of theformula where n and m may each independently be 1, 2, 3 or 4, up to twoof the carboxymethyl groups may be replaced with a p-hydroxyethyl groupand one or more of the carboxymethyl groups may be replaced bycarboxyethyl groups.

Since no two ferrous surfaces are likely to have the same amount ofcopper plated out thereon, it is advantageous that the copper-dissolvingferric chelate-containing solutions can be varied in concentration. Thestoichiometry of polycarboxylic chelating agents is wellknown and can beused to calculate the requirements for copper solution. In the case ofEDTA, for instance, one mole is required to solvate one mole of copper.Thus, as the plated copper is oxidized to copper ions, it reacts withfree chelating agent present as a salt or as an iron chelate.

The degree or precent of spentness of iron chelatechelating agentsolution is defined by:

Weight iron-complexed chelating agent Free chelating agent is determinedanalytically by a standard colorimetric or visual titration withstrontium chloride to a constant turbidity after first filtering thesample solution. From this analysis, the percent by weight of unchelatedchelating agent can be determined. Dissolved copper is analyticaldetermined by a chlorimetric or visual determination usingdiethyldithiocarbamate sodium salt as follows. Transfer ml. of samplesolution to a 250 ml. volume flask and dilute to volume with water. Mixsolution and transfer 5 ml. thereof to a beaker or flask. Add 1 ml. ofaqueous one percent diethyldithiocarbamate sodium salt and dilute toexactly 200 ml. volume with 2B ethanol. Mix solution thoroughly and takea reading on a colorimeter or take frequent samples and use the previoussample as a comparative blank to a constant visual end point. Totaldissolved iron and copper can be determined by X-ray emissionspectroscopy.

Most generally, optimum conditions for the removal 9 4 from the ferrousmetal surfaces, e.g., of boiler tubes, according to a procedurepreviously indicated.

Degree of spentness determines the corrosion rate at any giventemperature. The rate can be modified with iron-oxidation inhibitors. Ateach temperature, there is a degree of spentness above which there is nofurther corrosion. At about 140 F., it is about percent spentness, andat about 180 F., it is about 88 weight percent spentness, in the lattercase, if ca. 0.05 weight percent thioethylamine iron-oxidation inhibitoris present. At F., the degree of spentness can be reduced to 67 percentwithout any practical difference in corrosion rate. Theoretically, asolution containing more ferrous EDTA should require more oxygen. Theefficiency of copper oxidation is increased, however, and the sameamount or less air is required to oxidize the copper in a more highlyspent solution.

If the solution is 67 percent spent or more, no iron oxidation inhibitoris required at 140 F. At or F., the solution should be 91 percent ormore spent if no iron inhibitor or only an inhibitor as disclosed in US.Patent 3,077,454, is used.

The solubilized copper appears to be stabilized by the formation ofcupric chelate. Therefore, in highly spent solutions, the stripping ofcopper is accompanied by a reduction in the dissolved iron to give acolloidal ferric hydroxide precipitate.

After plated copper and iron oxide are removed from the treated ferroussurfaces, rising is accomplished, e.g., by draining the boiler andrefilling with water, all with air agitation. This facilitates theremoval of suspended undissolved solids and causes better rinsing.Finally, the rinse water is drained off. If the magnetite is notcompletely removed, small areas of copper are protected from oxidation.In contact with air and water, these areas develop tiny ant hills ofcorrosion products, i.e., red rust.

The following examples describe completely representative specificembodiments and the best mode contemplated by the inventors ofpracticing the invention. They are not to be taken as limiting theinvention other than as defined in the claims. Parts and percentagestherein are given by weight.

Example 1 A formulated spent solution of ammoniated EDTA was prepared byadding iron powder, in amount sufficient to saturate, to an aqueous 7.6weight percent solution of ammoniated EDTA originally adjusted to a pHof about 9 with free ammonia and maintained at a reaction temperature ofabout 95 C. for 30 minutes in the presence of a nitrogen atmosphere sothat the ferrous chelate of EDTA was formed. This solution was used toprepare a series of 3.8 percent total EDTA solutions having variouspercentages of spentness, e.g., by mixing with ammoniated EDTA adjustedto a pH of about 9 with free ammonia and with water, the percent ofspentness being measured as described above. About one-half gallon ofsuch solutions, some of them modified with a small percentage, up to ca.0.1 weight percent of an iron-oxidation inhibitor, were then placed in asimulated high pressure boiler containing 2.3-2.5 grams of plated copperon a square foot of inside surface. Simulated boiler heaters were turnedon to give various operating temperatures. Until a temperatureequilibrium was reached, a constant flow of nitrogen through thesimulated boiler was maintained to get circulation and exclude air. Thenitrogen purge was then changed to air, using a pressure regulator and avalve to meter air through a coarse frit at the bottom of the boilertubes. Time and rate of air flow were measured. The solutions weresampled for subsequent iron analysis, total iron and copper beingdetermined by X-ray emission spectroscopy. One or more steel couponswere suspended in the boiler tubes for corrosion data.

At pre-determined times, the steel coupons were removed and dried fordetermination of corrosion data and limited samples of the aqueoussolutions were taken. At the completion of each run, heat wasdiscontinued and the solution was drained from the bottom of the boiler.Distilled water was then added and the boiler tubes were air agitatedfor several minutes. The rinse water was drained and the boiler tubesremoved and examined. Operational data and results are summarized in thefollowing table.

TABLE I.-SUMMARY OF AIR-BLOWING DATA Percent Air-Blow Copper Inhibitor 1A. EDTA Temp,

Spent 2 F. Total, C.F.M. Required Avg, Percent min. Mins. mils StrippedRun No 1 11-124 plus ThEA 88 180 40 07 100 2 .(10 89. 5 180 035 100 98.5 180 40 035 100 91 180 004 100 83 140 125 004 100 140 180 004 100 67140 180 004 100 67 140 210 004 100 85 140 40 035 100 140 40 035 91. 0180 40 035 100 90 160 40 035 100 79 80 80 035 100 68. 5 140 240 004 10067. 0 140 30 .035 100 70. 0 140 40 035 99+ 86. 0 180 40 035 100 70.0 14030 035 100 82. 0 180 40 035 100 90. 2 180 40 035 100 86. 5 180 40 035100 81. 2 145 035 100 Final Gone. in Solution (Theoretical) OColrRrosion ate, Percent Cu Percent Fe Required Used Percentlbs/itfl/day (lb. moles) 00035 None None None None 0005 00034 0001300005 0002 None 0054 None None None None 0189 None 0330 0089 0403 0233 1A-124, an iron-oxidation inhibitor disclosed in U.S. Patent 3,077,454,is used in amount of 0.1%. ThEA is thloethylamine used in amount of0.05%.

2 Ammoniated EDTA, pH ca. 9.

3 The calculated theoretical no. oi mins. at the designated flow rate toconvert the ferrous EDTA to ferric EDTA and Cu to Cu ion.

Elemental analysis of the solutions in runs 1, 2 and 3 indicated thatthe copper was being complexed by the EDTA. In the case of highly spentsolvent (98.5 percent) the formation of tferric hydroxide was obvious.In other runs, the colloidal sediment could only be detected by thedifierence in iron analysis in filtered and unfiltered solutions.

Free ammonia content is :not critical. At the completion of run 3, forexample, the pH was almost neutral.

Runs 9 and 10 established the corrosion rate at F. when A124 was theonly inhibitor employed. The data indicate that a spentness of 6-7percent gave a negligible iron corrosion rate.

Run 13 indicated that, although there was no iron corrosion at 80 F. and79 percent spentness, the copper stripping rate was slow.

Example 2 In accordance with the method described in Example 1, thefollowing named polycarboxylic acid chelating agents were prepared as 80percent spent solutions and surfaces. Analytical procedures were thesame as in Example 1.

Polycarboxylic Acid Time to Strip Oil 100% of Scale, mins.

Example 3 through the solution in contact with a 0.25 mil copperscale ona ferrous surface.

Conc. EDTA, percent: Copper-strip 38 Yes.

4 Yes.

2 Yes. 0.5 Yes. 0.25 Partial.

At the 0.25 percent level, corrosion of the substrate was accelerated atstress areas.

Example 4 The following listed amines have been found to be operable inthe practice of this invention for the purpose of adjusting the pH tothe proper range and thereby forming the salts or partial salts ofpolycarboxylic acids and/or their partial salts, thereafter spending atleast a portion of the so-formed solution of chelating agent by addingiron to form iron chelate. These highly spent solutions are operable inremoving copper-containing scale from ferrous surfaces while havinglittle or no corrosive effect on the substrate, they also gave apassivated ferrous surface.

Amines: Used with: polycarboxylic acid Ethanolamine EDTA.

Ethylamine EDTA.

Ethylenediamine EDTA.

Diethylenetriamine EDTA.

Pentaethylenehexamine EDTA.

Dimethylamine EDTA.

Trimethylamine EDTA.

Ethyleneimine EDTA.

Ethanolamine Ethylenediaminetetrapropionic acid.

Ethylenediamine N,N-di-([i-hydroxyethyl) glycine.

Ammonia Tetramethylenediamine- N,N,N,N'-tetraacetic acid.

Ammonia (2-hydroxyethylimino) diacetic acid.

What is claimed is:

1. A process for removing copper from a ferrous metal surface containingcopper thereon by contacting said surface with an aqueous alkalinesolution wherein the solution employed contains as an essentialconstituent at least one member of the group consisting of ferricchelates of polycarboxylic acid chelating agents and mixtures 'of ferricand ferrous chelates of polycarboxylic acid chelating agents in amountsuflicient and for a time sufficient at a reaction temperature aboveabout 68 F. and up to about 300 F., to dissolve said copper.

2. A process as claimed in claim 1 wherein the said solution may alsocontain a salt of the group consisting of ammonium, amine andhydroxyalkylamine salts of polycarboxylic acid chelating agents.

3. A process as claimed in claim 2 wherein the total iron chelateoriginally present ranges between about and about 100 weight percent oftotal salt form and iron chelate and the solution originally contains atotal of between ca. 0.5 Weight percent and up to a saturated solutionof salt form and iron chelated chelating agent.

4. A process as claimed in claim 3 wherein some iron chelate ismaintained in the ferric form by the addition of a water-solublecompatible oxidizing agent.

5. A process as claimed in claim 3 wherein some iron chelate ismaintained in the ferri form by bubbling air through said aqueousalkaline solution.

6. A process as claimed in claim 3 wherein the iron chelate andchelating agent salt are those of ethylenediaminetetraacetic acid.

7. A process as claimed in claim 3 wherein the reacted ferrous surfaceis rinsed with rinse water containing a water-soluble oxidant.

8. A process as claimed in claim 3 wherein the reacted ferrous surfaceis rinsed with rinse water while air is bubbled therethrough.

9. A process for removing copper from a ferrous metal surface containingcopper thereon by contacting said surface with an aqueous alkalinesolution wherein the solution contains as an essential constituentammoniated ethylenediaminetetraacetic acid adjusted to a pH of about 9with free ammonia and iron chelate thereof wherein the iron chelateoriginally present ranges between about 60 and ca. 100 weight percent oftotal salt form ethylenediaminetetraacetic acid and iron chelate thereofand the solution originally contains between about 0.5 weight percentand up to a saturated solution of total salt form and iron chelate ofethylenediaminetetraacetic acid while air is being bubbled therethroughat a reaction temperature above about 68 F. and up to about 300 F. inamount suificient and for a time sufficient to dissolve said copper,removing said solution from said ferrous metal surface, contacting saidsurface with rinse water while bubbling air therethrough and removingsaid rinse water.

References Cited UNITED STATES PATENTS 2,396,938 3/1946 Bersworth 13422,959,555 11/1960 Martin et al. 134-41 3,308,065 3/1967 Lesinski 25282L. DEWAYNE RUTLEDGE, Pl'ilfllll'y Examiner.

T. R. FRYE, Assistant Examiner.

U.S. Cl. X.R.

1. A PROCESS FOR REMOVING COPPER FROM A FERROUS METAL SURFACE CONTAININGCOPPER THEREON BY CONTACTING SAID SURFACE WITH AN AQUEOUS ALKALINESOLUTION WHEREIN THE SOLUTION EMPLOYED CONTAINS AS AN ESSENTIALCONSTITUENT AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF FERRICCHELATES OF POLYCARBOXYLIC ACID CHELATING AGENTS AND MIXTURES OF FERRICAND FERROUS CHELATES OF POLYCARBOXYLIC ACID CHELATING AGENTS IN AMOUNTSUFFICIENT AND FOR A TIME SUFFICIENT AT A REACTION TEMPERATURE ABOVEABOUT 68*F. AND UP TO ABOUT 300*F., TO DISSOLVE SAID COPPER.